KR101585311B1 - Method for manufacturing high strength galvanized steel sheet having excellent stability of mechanical properties, formability, and coating appearance - Google Patents

Abstract

A method for manufacturing a high strength hot-dip galvanized steel sheet having a tensile strength (TS) of 540 MPa or more and excellent in material stability, workability, and plating appearance.
0.1% or less of C, 0.7% or more and 2.3% or less of Si, 0.8% or more and 2.0% or less of Mn, for steel sheet containing less than _{0.008%, O 2: 0.1 ~} 20 vol%, H 2 O: heated to 400 ~ 750 ℃ among 1 ~ 50 vol% atmosphere of, followed by O _2: 0.01 ~ less than 0.1 vol%, H ₂ O: 1 to 20 vol% in an atmosphere of H ₂ : 1 to 50 vol% and a dew point of 0 ° C. or less in a first heating step of heating the steel sheet at 600 to 850 ° C. in an atmosphere of 1 to 20 vol% At 15 to 600 seconds, cooled to a temperature range of 450 to 550 DEG C, and then subjected to a second heating step in which the temperature is maintained for 10 to 200 seconds, followed by hot-dip galvanizing.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a galvanized steel sheet for use in a galvanized steel sheet, which is excellent in material stability, processability, and plating appearance. BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a galvanized galvanized steel sheet,

The present invention relates to a method for producing a high-strength hot-dip galvanized steel sheet which is excellent in material stability and processability as well as excellent in plating appearance, as a member used in industrial fields such as automobiles and electric power.

In recent years, from the standpoint of global environmental conservation, improvement of fuel efficiency of automobile becomes important problem. Along with this, there has been an increase in the strength of the car body material to make it thinner and to make the car body itself lighter.

However, the increase in the strength of the steel sheet leads to a decrease in ductility, that is, a decrease in molding processability. Therefore, development of a material having high strength and high porosity has been desired.

Further, when forming a high-strength steel sheet into a complicated shape such as an automobile part, cracks and necking occur at a protruding portion and a stretching flange portion. Therefore, a high-strength steel sheet having both high strength and high hole expandability capable of overcoming cracking and necking problems is also required.

Further, the shape freezing property is remarkably lowered due to the high strength and thinning of the steel sheet. In order to cope with this, it has been widely practiced to predetermine a shape change after releasing at the time of press forming, and to design a die in anticipation of a shape change amount. However, when the tensile strength TS of the steel sheet is changed, the difference from the predetermined amount is increased and the shape defect is generated, and it is indispensable to modify the shape of one sheet after the press molding so that the mass productivity . Therefore, it is required that the deviation of the TS of the steel sheet be as small as possible.

As for the improvement of the formability of high strength steel sheets, various combinations of ferrite-martensitic two-phase steels and TRIP steels using the transformation induced plasticity of retained austenite have been used so far A tissue type high strength hot-dip galvanized steel sheet has been developed.

For example, Patent Document 1 proposes a method of manufacturing a galvanized steel sheet excellent in ductility, in which a chemical composition is specified in a specific range, a volume percentage of retained austenite and martensite, and manufacturing conditions are specified have. In addition, Patent Document 2 proposes a hot-dip galvanized steel sheet excellent in ductility, in which a chemical composition is specified in a specific range and a specific production condition is specified. Patent Document 3 proposes a galvannealed steel sheet excellent in ductility in which the chemical composition is specified in a specific range and the volume ratio of ferrite, bainitic ferrite and retained austenite is specified within a specific range. Patent Document 4 proposes a method of manufacturing a high-strength cold-rolled steel sheet having ferrite, bainite, and residual austenite of 3% or more and having improved deviation in elongation in the plate width direction.

Japanese Patent Application Laid-Open No. 2001-140022 Japanese Patent Application Laid-Open No. 04-026744 Japanese Patent Application Laid-Open No. 2007-182625 Japanese Patent Application Laid-Open No. 2000-212684

However, in Patent Documents 1 to 3, the main purpose is to improve the ductility of the high strength steel sheet, and the hole expandability is not considered. In Patent Document 4, only the deviation of the total elongation EL in the plate width direction is described, and the deviation of the material due to the component composition and the manufacturing conditions is not considered. As described above, in any of the techniques, a high-strength hot-dip galvanized steel sheet having both high strength and high hole expandability and excellent material stability is still not obtained.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances and has an object of providing a high strength hot-dip galvanized steel sheet having a tensile strength (TS) of 540 MPa or more and excellent in material stability, workability And a method of manufacturing the same.

The inventors of the present invention have conducted intensive studies to obtain a high-strength hot-dip galvanized steel sheet having a tensile strength (TS) of 540 MPa or more and excellent in material stability, workability (high strength and high hole expandability) .

Si is positively added so that the content thereof is made to be a predetermined amount or more, it is possible to improve the ductility by improving the work hardening ability of the ferrite, securing the strength by strengthening the solid solution of the ferrite and reducing the hardness difference with the second phase Improvement is possible.

By utilizing bainitic ferrite or pearlite, the hardness difference between soft ferrite and hard martensite can be alleviated, and hole expandability can be improved.

If a large amount of hard martensite is present in the final structure, a large difference in hardness is generated at the interface between the soft phase and the ferrite phase, so that the hole expandability is lowered. Therefore, the untransformed austenite finally transformed into martensite is called pearlite By making a structure having ferrite, bainitic ferrite, pearlite and a small amount of martensite, it is possible to improve hole expandability while maintaining high ductility, and by appropriately controlling the area ratio of each phase, The material stability can be ensured.

On the other hand, it is known that the plating appearance deteriorates when Si is contained. Usually, the hot-dip galvanized steel sheet is subjected to heat treatment in a reducing atmosphere and then subjected to hot-dip galvanizing treatment. Here, because Si added in the steel is an easily oxidizable element, it is selectively oxidized even in a reducing atmosphere generally used and is concentrated on the surface of the steel sheet to form an oxide. This oxide lowers the wettability with molten zinc in the plating process, causing unplated, so that the wettability is rapidly lowered along with the increase of the Si concentration in the steel, resulting in a plentiful non-plating.

With respect to such a problem, wettability with molten zinc can be improved by heating the steel sheet in advance in an oxidizing atmosphere to form iron oxide on the surface and then performing reduction annealing. On the other hand, on the other hand, in the initial stage of the reduction annealing, the iron oxide peeled off from the surface of the steel sheet adheres to the roll, which may cause a pressing scratch on the surface of the steel sheet. As a result of investigating to solve the problem of peeling of iron oxide from the surface of the steel sheet, it has been found that the peeling of iron oxide is suppressed by reducing the outermost surface of the iron oxide by heating the steel sheet in a microoxidative atmosphere after forming the iron oxide Respectively.

The present invention has been made based on the above findings, and its gist of the invention is as follows.

(1) C: 0.04 to 0.13%, Si: 0.7 to 2.3%, Mn: 0.8 to 2.0%, P: 0.1% or less, S: 0.01% , N: about 0.008% or less of the steel sheet containing, and comprising a balance of Fe and unavoidable impurities to, according to a front _{O 2: 0.1 ~ 20 vol%} , H 2 O: from 1 to an atmosphere containing 50 vol% that The steel sheet is heated at a temperature within a range of 600 to 850 ° C in an atmosphere containing O ₂ : less than 0.01 to 0.1 vol% and H ₂ O: 1 to 20 vol% And then the steel sheet is maintained in the temperature range of 750 to 900 DEG C for 15 to 600 seconds in an atmosphere containing H ₂ : 1 to 50 vol% and a dew point of 0 DEG C or lower, and 450 After cooling to a temperature range of ~ 550 ° C, it is maintained at a temperature range of 450 ~ 550 ° C for 10 ~ 200 s A second heating step and then a hot dip galvanizing treatment to obtain a ferritic steel sheet having a ferritic phase of 75% or more, a bainitic ferrite phase of 1.0% or more, a pearlite phase of 1.0% or more and 10.0% Plated steel sheet satisfying an area ratio of 1.0 to less than 5.0% and a martensite area ratio / (bainitic ferrite area ratio + pearlite area ratio) of 0.6 is obtained. , A processability and a galvanized steel sheet excellent in appearance.

(2) The steel sheet according to any one of (1) to (3), further comprising at least one member selected from the group consisting of at least one member selected from the group consisting of 1.0% or less of Cr, 0.5% or less of V, 0.5% or less of Mo, 1.0% or less of Ni, A method for producing a high strength hot-dip galvanized steel sheet excellent in material stability, workability and plating appearance described in (1) above, characterized by containing an element.

(3) The steel sheet according to the above (1), further comprising at least one element selected from the group consisting of Ti in an amount of 0.1% or less, Nb in an amount of 0.1% or less, and B in an amount of 0.0050% Or the high-strength hot-dip galvanized steel sheet excellent in material stability, workability and plating appearance described in (2).

(4) The steel sheet according to any one of (1) to (3) above, further comprising at least one element selected from the group consisting of Ca in an amount of not more than 0.005% and REM in an amount of 0.005% A method for producing a high strength hot-dip galvanized steel sheet excellent in material stability, workability and plating appearance described in one.

(5) The method according to any one of the above items (1) to (4), wherein the front end of the first heating step is carried out at a temperature of 1 to 1.3, (1) to (4), wherein the material stability, the workability, and the plating appearance are excellent.

(6) The method according to any one of (1) to (5) above, wherein after the hot dip galvanizing treatment, galvanization is performed under the conditions satisfying the following equation at a temperature range of 500 to 600 ° C A process for producing a high strength hot-dip galvanized steel sheet excellent in material stability, workability and plating appearance described.

0.45? Exp [200 / (400-T)] ln (t)? 1.0

only,

T: average holding temperature (° C) at a temperature range of 500 to 600 ° C

t: Holding time (s) in the temperature range of 500 to 600 DEG C

exp (X) and ln (X) represent the exponential function, natural logarithm of X, respectively.

In the present specification, the percentages representing the steel components are all% by mass. In the present invention, the "high strength hot-dip galvanized steel sheet" is a hot-dip galvanized steel sheet having a tensile strength (TS) of 540 MPa or more.

In the present invention, regardless of whether or not the alloying treatment is carried out, the steel sheet obtained by plating zinc on the steel sheet by hot-dip galvanizing is collectively referred to as hot-dip galvanized steel sheet. That is, the hot-dip galvanized steel sheet according to the present invention includes both hot-dip galvanized steel sheets not subjected to alloying treatment and galvannealed hot-dip galvanized steel sheets subjected to alloying treatment.

According to the present invention, a high-strength hot-dip galvanized steel sheet having a tensile strength (TS) of 540 MPa or more and excellent in workability and material stability and excellent in plating appearance can be obtained in terms of high ductility and high hole expandability. By applying the high-strength hot-dip galvanized steel sheet of the present invention to, for example, an automobile structural member, the fuel economy can be improved by reducing the weight of the vehicle body, and the industrial utility value is very high.

1 is a diagram showing the relationship between the annealing temperature (T ₁ ) and TS.
2 is a graph showing the relationship between the annealing temperature T ₁ and EL.
3 is a graph showing the relationship between the cooling average holding temperature (T ₂ ) and TS.
4 is a graph showing the relationship between the cooling average holding temperature (T ₂ ) and EL.

Hereinafter, the present invention will be described in detail.

(1) First, the composition of the components will be described.

(a) C: not less than 0.04% and not more than 0.13%

C is an austenite generating element and an indispensable element for strengthening steel. When the C content is less than 0.04%, it is difficult to secure the desired strength. On the other hand, if the C content exceeds 0.13%, hardening of the welded portion and the heat affected portion becomes remarkable and the mechanical properties of the welded portion deteriorate, resulting in deterioration of spot weldability, arc weldability and the like. Therefore, the C content is 0.04% or more and 0.13% or less.

(b) Si: not less than 0.7% and not more than 2.3%

Si is a ferrite-forming element, and is also an element effective for solid-solution strengthening. In order to secure good ductility by improving the work hardening ability on the ferrite phase, it is necessary to contain Si at 0.7% or more. In order to ensure the area ratio of the desired bainitic ferrite phase and to secure good hole expandability, it is also necessary to contain 0.7% or more of the bainitic ferrite phase. However, if Si is excessively contained, generation of a red scale or the like causes deterioration of the surface property and deterioration of adhesion and adhesion of plating. Therefore, the Si content is set to 0.7% or more and 2.3% or less. Preferably, it is 1.2% or more and 1.8% or less.

(c) Mn: not less than 0.8% and not more than 2.0%

Mn is an effective element for strengthening the steel. It is also an element that stabilizes austenite and is an element necessary for the fractional adjustment of the second phase. For this reason, Mn should be contained in an amount of 0.8% or more. On the other hand, if it is contained in excess of 2.0%, the area ratio of martensite in the second phase increases, and it becomes difficult to secure the material stability. Moreover, since the cost of alloy of Mn is recently rising, it is also connected to a factor of cost increase. Therefore, the Mn content is set to 0.8% or more and 2.0% or less. , Preferably not less than 1.0% and not more than 1.8%.

(d) P: not more than 0.1%

P is an effective element for strengthening steel, but if it is contained in an excess amount exceeding 0.1%, it causes brittleness due to grain boundary segregation and deteriorates impact resistance. If the content exceeds 0.1%, the alloying speed is greatly delayed. Therefore, the P content should be 0.1% or less.

(e) S: 0.01% or less

S is an inclusion of MnS or the like and causes cracking along with deterioration of impact resistance and metal flow of the welded portion. Therefore, the content is preferably as low as possible, but the S content is made 0.01% or less from the viewpoint of production cost.

(f) Al: not more than 0.1%

When the content of Al exceeds 0.1%, coarse Al ₂ O ₃ is produced and the material deteriorates. Therefore, the Al content is set to 0.1% or less. When Al is added for deoxidation of steel and the content thereof is less than 0.01%, coarse oxides such as Mn and Si are dispersed in the steel in a large amount to deteriorate the material, so that the content is preferably 0.01% or more . Therefore, the preferable range of the Al content is 0.01 to 0.1%.

(g) N: 0.008% or less

N is the element that deteriorates the corrosion resistance of steel to the greatest extent, and is preferably as small as possible. When the content is more than 0.008%, deterioration of endurance is remarkable. Therefore, the N content should be 0.008% or less.

The remainder is Fe and inevitable impurities. However, in addition to these elements, at least one selected from the following elements may be added as needed.

(h) at least one member selected from the group consisting of Cr: 1.0% or less, V: 0.5% or less, Mo: 0.5% or less, Ni: 1.0%

Cr, V, and Mo have an effect of improving the balance between strength and ductility, and can be added as needed. However, when the amount of Cr exceeded 1.0%, V: 0.5%, and Mo: 0.5% in excess, the fraction of the second phase would be excessively large, resulting in a significant increase in strength and the like. In addition, the cost may be increased. Therefore, when these elements are added, the amounts thereof are set to 1.0% or less of Cr, 0.5% or less of V, and 0.5% or less of Mo, respectively. In order to effectively exhibit the above effect, it is preferable that Cr: 0.05% or more, V: 0.005% or more, and Mo: 0.005% or more.

Ni and Cu are effective elements for strengthening the steel and can be added as needed. And also has an action of promoting the internal oxidation to improve the plating adhesion. However, if both Ni and Cu are contained in an amount exceeding 1.0%, the workability of the steel sheet is lowered. In addition, the cost may be increased. Therefore, when Ni and Cu are added, their contents are each 1.0% or less. In order to effectively exhibit the above effect, the content of Ni and Cu is preferably 0.05% or more.

(i) at least one selected from Ti: not more than 0.1%, Nb: not more than 0.1%, and B: not more than 0.0050%

Ti and Nb are effective elements for precipitation strengthening of the steel and can be added as needed. However, if the content exceeds 0.1%, workability and shape durability are deteriorated. In addition, the cost may be increased. Therefore, when Ti and NB are added, their contents are respectively 0.1% or less. In order to effectively exhibit the above effect, the content of Ti and Nb is preferably 0.01% or more.

B has an effect of inhibiting the formation and growth of ferrite from the austenite grain boundaries, and therefore can be added as needed. However, if it exceeds 0.0050%, the workability is lowered. In addition, the cost may be increased. Therefore, when B is added, its content is made 0.0050% or less. In order to effectively exhibit the above effect, the content thereof is preferably 0.0003% or more.

(j) at least one member selected from the group consisting of 0.005% or less of Ca and 0.005% or less of REM

Ca and REM (Rare Earth Metal) are effective elements for improving the adverse effect of sulfide on hole expandability by spheroidizing the shape of sulfide. However, if it is contained in an excessive amount, inclusions and the like are increased to cause surface and internal defects. Therefore, when Ca and REM are added, their content is 0.005% or less, respectively. In order to effectively exhibit the above effect, the content thereof is preferably 0.001% or more.

(2) Next, the steel structure will be described.

(a) Area ratio of ferrite phase: 75% or more

In order to secure good ductility, the ferrite phase is required to have an area ratio of 75% or more.

(b) Area ratio of bainitic ferrite phase: 1.0% or more

In order to secure good hole expandability, it is necessary to alleviate the hardness difference between soft ferrite and hard martensite, and therefore, the bainitic ferrite phase needs to be 1.0% or more in area percentage.

(c) Percentage area of perlite phase: not less than 1.0% and not more than 10.0%

In order to secure good hole expandability, the area ratio of the pearlite phase is set to 1.0% or more. In order to ensure a desired strength-ductility balance, the area ratio of the pearlite phase is set to 10.0% or less.

(d) Area ratio of martensite: not less than 1.0% and not more than 5.0%

In order to ensure the desired strength-ductility balance, the area ratio of the martensite should be 1.0% or more. In order to ensure good material stability, the area ratio of the martensite which greatly affects the tensile properties (TS, EL) needs to be less than 5.0%.

(e) martensite area ratio / (bainitic ferrite area ratio + pearlite area ratio)? 0.6

In order to ensure good material stability, it is preferable that the upper phase structure of the second phase is formed by reducing the amount of martensite which is a factor of the material deviation and increasing the amount of bainitic ferrite or pearlite which is softer than martensite, Martensite area ratio / (bainitic ferrite area ratio + pearlite area ratio) ≤ 0.6.

In addition to ferrite, bainitic ferrite, pearlite, and martensite, carbides such as retained austenite, tempered martensite, and cementite may be produced in some cases. However, the ferrite, bainitic ferrite, The object of the present invention can be achieved.

The area ratio of ferrite, bainitic ferrite, pearlite and martensite in the present invention means the area ratio of each phase occupied in the observation area.

The high-strength hot-dip galvanized steel sheet of the present invention is a high-strength hot-dip galvanized steel sheet comprising a steel sheet having the above-mentioned composition and the steel structure as a base steel sheet, and a plated film formed by hot-dip galvanizing or a galvanizing- Respectively.

(3) Next, the manufacturing conditions will be described.

The high-strength hot-dip galvanized steel sheet of the present invention is obtained by subjecting a steel sheet obtained from a steel having a component composition suitable for the above-mentioned component composition ranges to heat treatment in the following two steps and then performing hot-dip galvanizing or Followed by galvannealing after hot dip galvanizing.

(a) Production of steel sheet

The steel having the above-mentioned composition is subjected to solvent (melting) by a known method and then subjected to crushing or continuous casting to form a slab and hot-rolled to obtain a hot-rolled steel sheet. When the hot rolling is performed, it is preferable to heat the slab at 1100 to 1300 캜, perform hot rolling at a final finishing temperature of 850 캜 or higher, and wind the steel at 400 to 650 캜. When the coiling temperature exceeds 650 ° C, the carbides in the hot-rolled sheet are coarsened, and the coarsened carbides do not melt during the cracking at the time of annealing, so that the required strength may not be obtained. Thereafter, pickling treatment is carried out by a known method. The hot-rolled steel sheet obtained as described above may be used as the steel sheet, or a cold-rolled steel sheet obtained by further cold-rolling the hot-rolled steel sheet subjected to pickling may be used as the steel sheet. When the cold rolling is carried out, the conditions are not particularly limited, but it is preferable to carry out cold rolling at a cold reduction of 30% or more. If the cold reduction ratio is low, the recrystallization of the ferrite is not promoted and the unrecrystallized ferrite remains, resulting in deterioration of ductility and hole expandability.

(b) Heat treatment

(i) a first heating step

The first heating step is a step, in the front end, O _2: a rear end, and heating such that the 1 ~ 50 vol% of the steel sheet in an atmosphere temperature in the range of 400 ~ 750 ℃ containing,: 0.1 ~ 20 vol%, H 2 O , The steel sheet is heated to a temperature within the range of 600 to 850 ° C in an atmosphere containing O ₂ : less than 0.01 to 0.1 vol% and H ₂ O: 1 to 20 vol%.

· Shearing of the first heating process

The front end of the first heating step is carried out to oxidize the steel sheet, and O ₂ is required to be 0.1 vol% or more since it is necessary to sufficiently oxidize the steel sheet. For economic reasons, O ₂ is preferably 20 vol% or less of the atmospheric level. H ₂ O should be at least 1 vol% to promote oxidation. In consideration of the humidifying cost, H ₂ O is preferably 50 vol% or less. When the temperature after the heating in the shearing step is less than 400 ° C, the steel sheet is difficult to be oxidized. When the temperature exceeds 750 ° C, the steel sheet is excessively oxidized and the iron oxide is peeled off by the roll in the second heating step. Heat it.

· After the first heating step

The subsequent stage of the first heating step is performed in order to reduce the surface of the oxidized steel sheet once and suppress the pressure scratches. Therefore, the surface of the steel sheet once oxidized at the front end is subjected to heat treatment under the condition that the surface of the steel sheet can be subjected to reduction treatment and the iron oxide does not peel off, that is, under the conditions of low- The reducing treatment is carried out to the extent that the peeling of the iron oxide does not occur in the following second heating step. Since at this time the O ₂ can not be reduced more than in the 0.1 vol% O ₂ is less than 0.1 vol%. However, it is required to be 0.01 vol% or more. When H ₂ O is contained in a large amount, the steel sheet is oxidized, so it should be 20 vol% or less. However, at least 1 vol% of H ₂ O is required. When the steel sheet temperature is less than 600 ° C, it is difficult to reduce the steel sheet temperature. When the steel sheet temperature exceeds 850 ° C, the heating cost is increased, so that the steel sheet is heated so that the steel sheet temperature is within the range of 600 ° C to 850 ° C.

In the case where the shearing heating is carried out by the direct-current method (DFF) or the non-oxidizing furnace (NOF), it is preferable that the combustion gas is C gas generated from the coke oven and the air ratio is 1 to 1.3. This is because, if the air ratio is less than 1, the steel sheet is not oxidized. If the air ratio is more than 1.3, pick-up occurs due to over-oxidation. In the case where the rear end heating is carried out by direct-injection (DFF) or non-oxidizing furnace (NOF), it is preferable to use C gas generated in the coke oven as the combustion gas, . This is because if the air ratio is 1 or more, the iron oxide on the surface of the steel sheet can not be reduced, and if the air ratio is less than 0.6, the combustion efficiency is deteriorated.

(Ii) a second heating step

The second heating step is carried out after the first heating step to perform the reduction treatment and the adjustment of the steel sheet texture. The second heating step is carried out in an atmosphere of H ₂ : 1 to 50 vol% To 900 ° C for 15 to 600 s, cooled to a temperature of 450 to 550 ° C, and maintained at a temperature of 450 to 550 ° C for 10 to 200 s.

· H ₂ : an atmosphere containing 1 to 50 vol% and having a dew point of 0 ° C or less

If H ₂ is less than 1 vol% and the dew point is more than 0 ° C, the iron oxide produced in the first heating step is not reduced well, so that even if iron oxide is generated enough to secure the plating property in the first heating step, The plating ability is deteriorated. When H ₂ exceeds 50 vol%, the cost increases. When the dew point is less than -60 占 폚, it is difficult to industrially carry out the dew point. Therefore, the dew point is preferably -60 占 폚 or more.

· Maintain 15 ~ 600 s at the temperature range of 750 ~ 900 ℃

Annealing is performed in a temperature range of 750 to 900 占 폚, specifically, in austenite single phase region or a bipolar region of austenite and ferrite for 15 to 600 seconds. When the annealing temperature is less than 750 占 폚 or the holding time is less than 15 s, the hard cementite in the steel sheet is not sufficiently dissolved, the hole expandability is lowered and the desired martensite area ratio is not obtained, . On the other hand, if the annealing temperature exceeds 900 DEG C, the growth of the austenite grains is remarkable, and it becomes difficult to secure the bainitic ferrite due to the bainite transformation which occurs during the cooling after the cooling, The jit area ratio / (bainitic ferrite area ratio + pearlite area ratio) exceeds 0.6, so that good material stability can not be obtained. When the holding time exceeds 600 s, the austenite is coarsened, making it difficult to obtain a desired strength, and in some cases, an increase in cost due to a large energy consumption may be caused.

· Maintain 10-200 s at the temperature range of 450 ~ 550 ℃

After the above-mentioned annealing, the substrate is cooled to a temperature range of 450 to 550 ° C and maintained at a temperature of 450 to 550 ° C for 10 to 200 s. If the holding temperature exceeds 550 DEG C or the holding time is less than 10 s, the bainite transformation is not promoted and the area ratio of the bainitic ferrite becomes less than 1.0%. Therefore, the desired hole expandability can not be obtained. If the holding temperature is less than 450 占 폚 or the holding time exceeds 200 seconds, most of the second phase becomes austenite and bainitic ferrite having a large amount of solid carbon produced by the promotion of bainite transformation, The pearlite area ratio can not be obtained and the area ratio of the hard martensite is 5.0% or more. Therefore, good hole expandability and material stability can not be obtained.

(c) Hot dip galvanizing treatment

After the second heating step, the steel sheet is immersed in a plating bath of a conventional bath to perform hot dip galvanization, and the amount of plating adhered is adjusted by gas wiping or the like, thereby cooling the hot dip galvanized steel sheet without alloying the plating layer.

When producing a hot-dip galvanized steel sheet to be subjected to alloying treatment, hot-dip galvanizing is carried out, and galvanization of the zinc plating is carried out under the condition that the following formula is satisfied at a temperature range of 500 to 600 占 폚.

0.45? Exp [200 / (400-T)] ln (t)? 1.0

only,

T: average holding temperature (° C) at a temperature range of 500 to 600 ° C

t: Holding time (s) in the temperature range of 500 to 600 DEG C

exp (X) and ln (X) represent the exponential function, natural logarithm of X, respectively.

When the ratio of exp [200 / (400-T)] xln (t) is less than 0.45, martensite is present in a large amount in the steel structure after alloying treatment and the hard martensite is adjacent to soft ferrite, A large hardness difference is generated between them, and hole expandability is deteriorated. In addition, the martensitic area ratio / (bainitic ferrite area ratio + pearlite area ratio) exceeds 0.6, so that the material stability is impaired. Further, the adhesion of the hot-dip galvanized layer is deteriorated. When exp [200 / (400-T)] x ln (t) exceeds 1.0, most of untransformed austenite is transformed into cementite or pearlite, resulting in a balance of desired strength and ductility.

In addition, in the temperature range lower than 500 占 폚, the alloying of the plating layer is not promoted and it is difficult to obtain a galvannealed steel sheet. Further, at a temperature range exceeding 600 deg. C, most of the second phase becomes pearlite, and the desired martensite area ratio is not obtained, and the balance between strength and ductility is lowered.

Galvannealing treatment is performed so that exp [200 / (400-T)] ln (t) satisfies the above range at a temperature range of 500 to 600 ° C, Can be obtained.

According to the present invention as described above, a high-strength hot-dip galvanized steel sheet having a tensile strength (TS) of 540 MPa or more, excellent workability and material stability, and excellent plating appearance can be obtained.

1 and 2 are graphs showing the results obtained by comparing the results of Examples 15, 16 and 17 (Table 2 and Table 5) of Steel A, which is an example of the present invention to be described later, ) Is a diagram summarizing the relationship between the annealing temperature (T ₁ ) and TS in the second heating step and the relationship between the annealing temperature (T ₁ ) and EL. 3 and 4 are graphs showing the results obtained by comparing the results of Examples 21, 22 and 23 (Table 2 and Table 5) of Steel A, which is an example of the present invention to be described later, Table 5 shows the relationship between the average holding temperature (T ₂ ) of cooling after annealing in the second heating step and TS, and the relationship between the average holding temperature (T ₂ ) and EL.

1 and 2, it can be seen that the variation of TS and EL in the steel A of the present invention is small, while the variation of TS and EL in the steel H of the comparative example is large. 3 and 4, it can be seen that the fluctuation of TS and EL in the steel A of the present invention is small while the fluctuation of TS and EL in the steel H of the comparative example is large.

From the above results, it can be seen that a high-strength hot-dip galvanized steel sheet having high material stability can be obtained by the present invention.

In the series of heat treatment in the production method of the present invention, the holding temperature need not be constant if the temperature is within the above-mentioned range, and it may be within a prescribed range even when the cooling rate is changed during cooling. If the heat history specified in the present invention is satisfied, the steel sheet may be subjected to heat treatment by any facility. In addition, it is within the scope of the present invention to perform temper rolling on the steel sheet of the present invention for shape correction after heat treatment.

Further, the steel sheet of the present invention is typically manufactured through various steps such as steelmaking, casting, hot rolling, and the like. Typically, the steel sheet is partially or wholly omitted by, for example, thin casting .

[Example]

A steel having the composition shown in Table 1 and the balance of Fe and unavoidable impurities was converted into a slab by a continuous casting method. The obtained slab was heated to 1200 占 폚, hot rolled to a plate thickness of 3.2 mm at a finishing temperature of 870 to 920 占 폚, and then rolled at 520 占 폚. Then, the obtained hot rolled sheet was pickled to obtain a hot rolled steel sheet. Some of them were hot rolled steel sheets as pickled, and the remaining cold rolled steel sheets were cold rolled steel sheets. Subsequently, the hot-rolled steel sheet and the cold-rolled steel sheet as obtained in the pickling state as described above were subjected to annealing treatment under the manufacturing conditions shown in Tables 2 to 4 by a continuous hot-dip galvanizing line to carry out hot dip galvanizing treatment, To obtain a hot-dip galvanized steel sheet (cold-rolled steel sheet hot-dip galvanized material: Nos. 1 to 90, Hot-rolled steel sheet hot-dip galvanized material: Nos. 91 and 92). The plating amount was 30 to 50 g / m 2 per one side. A part of a hot-dip galvanized steel sheet not subjected to alloying treatment after the hot-dip galvanizing treatment was also produced.

The area ratio of the phase of ferrite, bainitic ferrite, pearlite and martensite to the obtained hot-dip galvanized steel sheet was obtained by abrading the plate thickness cross-section parallel to the rolling direction of the steel sheet with 3% Electron microscope) at a magnification of 2000 times and observed with a 10-field observation using Image-Pro of Media Cybernetics. At that time, since it is difficult to distinguish the martensite from the retained austenite, the obtained hot-dip galvanized steel sheet is tempered at 200 DEG C for 2 hours, and thereafter, The texture was observed by the above method, and the area ratio of the tempered martensite obtained by the above method was regarded as the area ratio of the martensite. The volume percentage of the retained austenite phase was obtained by grinding the steel sheet up to a quarter of the plate thickness direction and by the diffracted X-ray intensity of this plate thickness 1/4 plane. CoKα lines were used for the incident X-rays and the integrated intensities of the peaks of the {111}, {200}, {220}, and {311} planes of the retained austenite phase and the {110}, {200} And the average value thereof was defined as the volume percentage of the retained austenite phase.

The tensile test was carried out in accordance with JIS Z 2241 using a JIS No. 5 test piece in which a sample was taken such that the tensile direction was perpendicular to the rolling direction of the steel sheet and TS (tensile strength), EL (total elongation) Was measured, and it was judged that the ductility was good when TS EL ≥ 19000 ㎫ ·%.

Material stability is, (A) the annealing temperature for different steel Only the same as the annealing temperature T ₁ condition other than T _1, TS, investigated the variation of the EL, and the TS, the annealing temperature change per change amount 20 ℃ from the variation amount of the EL (ΔTS, ΔEL) to obtain, and (B) of the average holding temperature T the average holding temperature is different from the steel sheet, only T ₂ to the same conditions other than _2, and the plating bath is immersed after cooling to plating bath after cooling immersion, TS, (TS) and EL (EL) changes per 20 占 폚 per 20 占 폚, and the variation (? TS,? EL) per 20 占 폚 of the average holding temperature change from the cooling TS to the immersion And the variation amount (DELTA EL).

Further, for the hot-dip galvanized steel sheet obtained as described above, hole expandability (stretch flangeability) was measured. Hole expandability (elongation flangeability) was conducted in accordance with JFS T 1001 of Japan Steel Federation. After cutting each of the obtained steel plates to 100 mm x 100 mm, a hole having a diameter of 2.0 mm or more was formed with a clearance of 12% ± 1% and a plate having a thickness of less than 2.0 mm with a clearance of 12% ± 2% , A punch of a conical 60 ° was pushed into a hole in a state in which a dicing die having an inner diameter of 75 mm was suppressed to 9ton and the hole diameter at the cracking occurrence limit was measured. (%), and the stretch flange formability was evaluated from the value of the limit hole expanding ratio, and the case where?? 70 (%) was judged as good.

Limit hole expansion factor λ (%) = {(D _f -D ₀ ) / D ₀ } × 100

D _f is the hole diameter (mm) at the time of cracking, and D ₀ is the initial hole diameter (mm).

The surface appearance was examined by the following method.

(?) And appearance defects were slightly observed. On the other hand, when there was no appearance defect, the appearance was generally good (?). When the appearance defect was unsatisfactory, (X).

The results obtained by the above are shown in Tables 5 to 7.

In the method of manufacturing a high strength hot-dip galvanized steel sheet according to the present invention, TS is not less than 540 MPa, λ is not less than 70%, excellent hole expandability, and TS × EL ≧ 19000 ㎫ ·% It can be seen that a high-strength hot-dip galvanized steel sheet excellent in workability is obtained. It can be also seen that a high strength hot-dip galvanized steel sheet having a small value of DELTA TS and DEL is obtained and excellent in material stability. On the other hand, in the comparative example, at least one of ductility and hole expandability is inferior or material stability is not preferable.

Further, the high strength hot-dip galvanized steel sheet of the present invention example is free of unplated and has excellent surface appearance, but in the comparative example, unplated occurs and the surface appearance is inferior.

Industrial availability

The high-strength hot-dip galvanized steel sheet of the present invention has a tensile strength (TS) of 540 MPa or more and also has high ductility, high hole expandability, and further excellent material stability. By applying the high-strength hot-dip galvanized steel sheet of the present invention to, for example, an automobile structural member, the fuel economy can be improved by reducing the weight of the vehicle body, and the industrial utility value is very high.

Claims (12)

At least 0.03% and not more than 0.13% of Si, at least 0.7% and at most 2.3% of Mn, at least 0.8% and at most 2.0% of P, at most 0.1% of P, at most 0.01% , N: 0.008% or less and the balance of Fe and inevitable impurities is heated to 1100 to 1300 ° C and hot-rolled at a final finishing temperature of 850 ° C or higher, and then rolled at 400 to 650 ° C , For a steel sheet obtained by pickling treatment,
In the front end _{O 2: 0.1 ~ 20 vol%} , H 2 O: 1 ~ O 2 in the rear end of the heating, and in an atmosphere containing 50 vol% so that a temperature in the range of 400 ~ 750 ℃: 0.01 ~ 0.1 vol% And H ₂ O: 1 to 20 vol%, so that the temperature is in a range of 600 to 850 ° C.
Subsequently, it is maintained in a temperature range of 750 to 900 ° C for 15 to 600 seconds in an atmosphere containing H ₂ : 1 to 50 vol% and a dew point of 0 ° C or lower, cooled to a temperature range of 450 to 550 ° C, A second heating step of maintaining the temperature at 550 DEG C for 10 seconds to 200 seconds,
A hot-dip galvanizing treatment is performed,
A ferrite phase of not less than 75%, a bainitic ferrite phase of not less than 1.0%, and a pearlite phase of not less than 1.0% and not more than 10.0% in an area ratio of not less than 1.0% and less than 5.0% Galvanized steel sheet satisfying the following formula (1): (1) a hot-dip galvanized steel sheet having an area ratio of gypsum / (area of bainitic ferrite + area ratio of pearlite) of 0.6. The method according to claim 1,
Wherein the steel sheet further contains at least one element selected from the group consisting of 1.0% or less of Cr, 0.5% or less of V, 0.5% or less of Mo, 1.0% or less of Ni and 1.0% or less of Cu Wherein the galvanized steel sheet has excellent material stability, workability and plating appearance. The method according to claim 1,
The steel sheet further comprises at least one element selected from the group consisting of Ti in an amount of 0.1% or less, Nb in an amount of 0.1% or less, and B in an amount of 0.0050% or less. A method of manufacturing this excellent high strength hot dip galvanized steel sheet. The method according to claim 1,
Wherein the steel sheet further contains at least one element selected from the group consisting of Ca in an amount of not more than 0.005% and REM in an amount of not more than 0.005% in terms of mass%, a high strength hot-dip galvanizing A method of manufacturing a steel sheet. The method according to claim 1,
Wherein the preheating of the first heating step is performed by a direct heating furnace or a non-oxidizing furnace under the condition that the air ratio is not less than 1 and not more than 1.3, and the end of the first heating step is a direct heating furnace or a non- Wherein the hot-dip galvanized steel sheet has excellent material stability, workability and plating appearance. 6. The method according to any one of claims 1 to 5,
Characterized in that, after the hot-dip galvanizing treatment, alloying treatment of zinc plating is performed under the condition that the following formula is satisfied at a temperature range of 500 to 600 占 폚 to obtain a high-strength hot-dip galvanized steel sheet excellent in material stability, Gt;
0.45? Exp [200 / (400-T)] ln (t)? 1.0
only,
T: average holding temperature (° C) at a temperature range of 500 to 600 ° C
t: Holding time (s) in the temperature range of 500 to 600 DEG C
exp (X) and ln (X) represent the exponential function, natural logarithm of X, respectively. 3. The method of claim 2,
The steel sheet further comprises at least one element selected from the group consisting of Ti in an amount of 0.1% or less, Nb in an amount of 0.1% or less, and B in an amount of 0.0050% or less. A method of manufacturing this excellent high strength hot dip galvanized steel sheet. 3. The method of claim 2,
Wherein the steel sheet further contains at least one element selected from the group consisting of Ca in an amount of not more than 0.005% and REM in an amount of not more than 0.005% in terms of mass%, a high strength hot-dip galvanizing A method of manufacturing a steel sheet. The method of claim 3,
Wherein the steel sheet further contains at least one element selected from the group consisting of Ca in an amount of not more than 0.005% and REM in an amount of not more than 0.005% in terms of mass%, a high strength hot-dip galvanizing A method of manufacturing a steel sheet. 3. The method of claim 2,
Wherein the preheating of the first heating step is performed by a direct heating furnace or a non-oxidizing furnace under the condition that the air ratio is not less than 1 and not more than 1.3, and the end of the first heating step is a direct heating furnace or a non- Wherein the hot-dip galvanized steel sheet has excellent material stability, workability and plating appearance. The method of claim 3,
Wherein the preheating of the first heating step is performed by a direct heating furnace or a non-oxidizing furnace under the condition that the air ratio is not less than 1 and not more than 1.3, and the end of the first heating step is a direct heating furnace or a non- Wherein the hot-dip galvanized steel sheet has excellent material stability, workability and plating appearance. 5. The method of claim 4,
Wherein the preheating of the first heating step is performed by a direct heating furnace or a non-oxidizing furnace under the condition that the air ratio is not less than 1 and not more than 1.3, and the end of the first heating step is a direct heating furnace or a non- Wherein the hot-dip galvanized steel sheet has excellent material stability, workability and plating appearance.

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